1. Field of the Invention
This invention relates generally to wireless digital communication systems. In particular, the invention discloses a technique for increasing the performance of a wireless communication link operating in the presence of an external source of intermittent interference.
2. Description of the Prior Art
Cordless telephones have become increasingly popular in recent years. As improved technology is incorporated into cordless telephone designs, their sound quality and reliability have greatly improved, leading to the increasing proliferation and acceptance of cordless telephone devices in residential, and even commercial, environments.
However, as cordless telephones (which operate via a wireless radio frequency (RF) communication link established between the phone handset and a base unit) and other wireless devices have become more popular, the electromagnetic spectrum over which such devices communicate has, in turn, become increasingly crowded. Additionally, increasing numbers of other electronic devices are being used throughout society. Many such devices radiate electromagnetic energy that “pollutes” the spectrum over which wireless devices must communicate. As a result of these spurious transmissions and radiated noise, wireless devices commonly experience crosstalk and interference that inhibits the accurate and reliable transmission of signals.
In order to reduce the crowding of the electromagnetic spectrum designated for use by cordless telephones and other personal wireless devices, numerous frequency domain interference avoidance techniques, such as dynamic channel allocation, have been developed. Such techniques typically involve altering of the radio frequencies corresponding to the “channels” over which a wireless device communicates in an attempt to avoid externally generated radiated electromagnetic energy. Also, additional frequency bands have been allocated for personal wireless communications devices, thereby spreading out device traffic and reducing the potential for interference.
Systems known in the art utilize many techniques to avoid the effects of RF interference that occur in a device's communication band. Many such techniques involve changing the system's frequency domain RF transmission characteristics. For example, calls may be handed off to a different carrier frequency, or noisy channels in the hopping pattern of a frequency hopping spread spectrum system may be substituted for different channels. Such channel substitution techniques are particularly effective in avoiding continuous narrowband sources of interference, such as may be generated by other cordless telephones.
Recently, the 2.4 GHz ISM band has become popular for use by cordless telephones and other wireless devices. Telephones operating in the 2.4 GHz ISM band commonly employ a Time Domain Multiple Access (“TDMA”) communication protocol comporting with the Digital Enhanced Cordless Telecommunications (“DECT”) standard, as defined in ETSI standard ETS 300 175-2, Section 4.2 (September 1996). Due to the characteristics and regulation of the 2.4 GHz band, devices operating thereon may offer increased range and/or bandwidth compared to other available frequency bands. The DECT standard is being widely adopted throughout the world for wireless applications including cordless telephones, wireless offices and wireless telephone lines to the home. The DECT standard allows for multiple communication links between devices on a single RF carrier frequency through the use of time domain multiplexing. Following the success of DECT in Europe, Africa and South America, a variant of DECT has been developed for the North American market called Worldwide Digital Cordless Telecommunications (WDCT). The WDCT standard is currently becoming popular for use at the 2.4 GHz ISM band.
However, one substantial difficulty faced by designers of electronic equipment utilizing the 2.4 GHz ISM band is interference generated by operation of a common household microwave oven. During their operation, microwave ovens generate substantial levels of RF energy throughout the 2.4 GHz frequency range. Therefore, when a microwave oven is in operation, a nearby, active 2.4 GHz cordless telephone of prior art design would commonly experience substantial interference. Such interference would degrade the sound quality of the telephone call to an objectionable, if not unusable, level. The impact of microwave oven radiation on cordless telephone operation is particularly significant when users place the cordless telephone base unit directly on top of a microwave oven. In certain circumstances, it is possible that a cordless telephone call conducted in the presence of microwave oven generated interference would be dropped altogether.
Prior art frequency domain interference avoidance techniques are of limited effectiveness in the presence of an interference source that emits interfering energy over a substantial portion of a communication band. It is estimated that a microwave oven may radiate substantial levels of interfering RF energy on a majority of the frequency channels defined in the 2.4 GHz ISM band.
Accordingly, it is an object of this invention to provide a method that can be used in conjunction with a wireless communications system to avoid electromagnetic interference radiated by a microwave oven, or similar source.
In addition to reliability and sound quality, power efficiency of a cordless telephone handset is an important consideration in cordless telephone design. Cordless telephone customers demand telephones with extended battery life, such that talk time and time between required charging of the telephone handset needs to be as great as possible. However, consumers also desire compact and light weight portable telephone handsets, which, in turn, limits the physical size and, in turn, the electrical capacity of the battery that may be incorporated. While compact, high energy density battery technologies are one solution, they tend to be expensive, thereby increasing the cost of a cordless telephone that uses high density batteries to extend talk time. Therefore, it is highly desirable, and therefore it is an object of this invention, to provide a cordless telephone design that is power efficient.
Finally, some advanced wireless communications systems utilize multiple communication links over a single time domain multiplexed data frame. For example, advanced cordless telephone base units may support multiple portable handsets; wireless data communications may involve multiple devices on different time slots of a common carrier, and Wireless Local Loop technology may provide wireless telephone line service to a plurality of handsets in one or more homes using a common RF carrier. Therefore, it is an object of this invention to provide an interference avoidance technique that efficiently utilizes the capacity of a communications channel.
These and other objects of this invention will become apparent to those of ordinary skill in the art in view of the invention described herein.